When sheets are transported through a printing machine, their printed side often bears against printing cylinders and/or drums. This tends to cause a smearing of the printed image if there is even the slightest relative motion between the sheet and the outer cylindrical surface. When both sides of a sheet are printed, it is even more difficult not to smear the image since an inked side will always bear against one of the cylinders, and any ink that has not fully dried will come into contact with the bearing surface. However, it is known that by reducing the total surface area where the sheet contacts the cylinder, smearing of sheets can be reduced. Accordingly, various attempts to reduce the total bearing area have been made to remedy the smearing problem.
For example, German Utility Model 18 31 636 attempts to solve the smearing problem by roughening the casing surfaces surrounding the cylinders with a knurling tool. The printed sheets thus are supported at the different height elevations of the casing surface. However, the sheet bears unevenly due to the different elevations, and ink tends to build up in the casing surface. Thus the smearing is ultimately increased, especially as the roughened casing surface becomes worn down over time.
Alternatively, German Patent no. DE 1,100,651 discloses an attempt to solve the smearing problem by providing a casing consisting of a firm fabric base and a special rubber layer into which a number of small glass beads are imbedded. The sheet then does not contact the cylinder directly, but instead bears on the beads.
U.S. Pat. Nos. 5,042,383, 4,327,135 and 4,688,784 (corresponding to German Patent nos. DE 2,446,188, DE 2,916,505 and DE 3,422,443 respectively) as well as German Patent No. DE 3,931,479 are directed to cylinders having hemispherical domes of equal height distributed over the casing surfaces or foil covers. The domes are chemically coated and are arranged to enable both airflow and ink runoff to prevent ink from building up (i.e., mantling) on the domed bearing surfaces.
However, although the mantling of ink is reduced with these aforementioned dome-shaped casings, when printing materials having a low paper weight are used, the sheets sag atop the domes, thus increasing the overall amount of sheet-to-dome bearing area (i.e., the total surface contact area). Smearing thus increases with lower-weight sheets. Moreover, wear and abrasion cause the upper contact areas of the domes to flatten out over time. Thus, over time, sheets bear against an exponentially increasing contact area, which results in a corresponding progressive increase in ink smearing.
Finally, German Patent DE 3,913,818 (assigned to the assignee of the present application), discloses an impression cylinder having a reticular screen structure of a predetermined screen fineness. Although ink mantling is reduced using this screened casing surface, the irregular lines of the screen comprise the raised bearing surface for the sheet. Since the screen is relatively fine, these lines provide a large total bearing surface, which again results in ink smearing. This is especially prevalent in two-sided printing operations.